Hi again,
> > a few month ago, Markus Mottl pointed to this mailing list
> the work by
> > Claudio Russo on first class modules. There were no answer
> about plans to
> > implement such a system for OCaml.
>
> Well, it seems like Russo's first-class modules could be added with
> relatively little effort, if there is a sufficient need for them.
> (In OCaml and also in Luc Maranget's Hevea, I can see the need for
> conditionally selecting between several structures having the same
> signature; first-class modules give almost this but not quite.)
Why can't you do this with first-class modules?
In Moscow, you can write (note the somewhat different syntax designed to
co-exist with SML):
structure X as S = if toss_coin() then [structure Foo as S] else
[structure Bar as S];
It's a little redundant, but works.
If you want this to appear as a top-level or nested structure
declaration, then this does rely on
integrating open with ordinary declarations, but this is
straightforward.
> > As I see it, the main issue is the unification problem < S
> > = < T >.
>
> Right. The last time we met, I asked Claudio about type inference
> issues for his scheme. Basically, to "unify" <S> and <T>, you just
> check that the module types S and T are equivalent (using the same
> notion of equivalence that OCaml currently uses to for checking
> compatibility between manifest module type declarations, see
> the predicate
> Includemod.check_modtype_equiv in the OCaml sources).
>
> If <S> and <T> contain value components with non-generalized type
> variables, it is necessary to unify them along the way, and Claudio
> alluded to potential traps here. However, I'm not even sure this can
> happen at all in OCaml since module types cannot contain n-g type vars
> and "pack" requires an explicit module type constraint.
This isn't quite true (unless you don't allow n-g vars in inferred
module types either).
Counter-example:
fun f x = [structure struct val y = x end as sig val y:int end];
> val f = fn : int -> [{val y : int}]
applying the constraint should affect the type of x (which will be a
free var until the
signature is matched against).
FYI, in Moscow you can even write,
fun f (x:'a) = [structure struct val y = x end as sig val y:'a end];
> val 'a f = fn : 'a -> [{val y : 'a}]
so that f is polymorphic.
>
> > (as a side effect, we get the local "open ... in ...")
>
> I'm not sure I follow you here. Did you mean that "open" and "pack"
> subsume "let module ... in ..."? This I agree with.
I think the best approach is the other way around, treating open as
another form of declaration.
Cheers,
Claudio
> - Xavier Leroy
>